In addition to a narrow-line emission spectrum, Seyfert 1s have broad permitted
lines (H I, He I
5876,6678,7065, He II
4686 and Fe II
in the visible domain).

The narrow-line spectra of Seyfert 1s are very similar to those of Seyfert 2s;
there are however significant differences, most notably much stronger high-
ionization lines ([Fe VII]6087,
[Fe X]6375,
[Fe XI]7892) in some Seyfert 1s
([85];
[110];
[375]).
[120]
showed that
these ``coronal lines'' form just outside the BLR, in clouds with
electron temperatures
T ~ 12 000-150 000 K and densities of 102-108.5
cm-3.

There seems to be an anticorrelation between R5007, the ratio
of the intensities of the broad
H component to the
[O III]5007 line and the
broad
H component
luminosity or the continuum luminosity, i.e. bright QSOs
have relatively weak narrow lines (since in the NLR R5007 ~
0.1, the measurement
of R5007 is a measurement of the ratio of the broad to the
narrow H component
fluxes). There are however probably no ``pure'' Seyfert 1s,
i.e. bare BLRs
without surrounding NLRs; the objects which come closest are the Seyfert
Mark231, in which the only identified narrow line is
[O II]3727 and
the QSO 3C273
([85]).

The decrease in the relative amount of narrow-line emission with luminosity
suggests either that the ionizing flux to the narrow-line region is
proportionately smaller in objects with a high-luminosity broad-line component,
or else that there is proportionately less low-density gas in the higher
luminosity Seyferts
([85]).

The broad emission lines observed in AGNs have a FWHM which is typically in the
range 5000-10000 km s-1 and show different kinds of profiles that
usually are not Gaussian or even symmetrical
([402];
[95]).
The half-width at zero intensity of
H can be extremely
large, reaching 35 000 km s-1 in the case of PG 0052+251
([55]).
Crenshaw (1986) showed that the
H /
H and He I
5876 /
H ratios increase
from the core
to the wings of the lines, indicating that the broad-line region is not a thin
spherical shell.

Woltjer (1959) was the first to suggest that the width of the lines
could be due to fast motions in the gravitational field of a massive nucleus.
Assuming the line-emitting matter is gravitationally bound, and hence has a
near-Keplerian velocity dispersion (indicated by the line width), it is
possible
to estimate the central mass. The main problem in estimating the mass
from the emission line data is to obtain a reliable estimate of the size of
the BLR; this size can be measured by reverberation mapping. Both the
continuum
and the broad emission line fluxes are variable in Seyfert 1s and QSOs;
the time
lags between the emission line and the continuum light curves can be
interpreted
in terms of the delayed response of the spatially extended BLR to the ionizing
continuum source. These observations have established that Seyfert 1 BLRs have
sizes of the order of a few light-days to light-months ( ~ 100 light-days).
The BLR size of these objects is consistent with the hypothesis that the BLR
size grows as L0.5 as expected if the shape of the ionizing
continuum in
AGNs is independent of the luminosity L, and that all AGNs are characterized by
the same ionization parameter and BLR gas density
([216];
[459]).
The H FWHM
correlates with the
H luminosity,
probably reflecting a scaling of the central mass with the luminosity
([293]).
Whether or not the broad emission line widths actually
reflect virial motions is still somewhat problematic; if however this is the
case, the BLR provides a definitive demonstration of the existence of
supermassive BHs. An estimate has been made by the reverberation mapping
method
of the BH mass in 19 Seyfert 1s; these masses range from 0.4 to
40 107
M
([461]).
The average black-hole-to-bulge mass ratio is 0.0003 for AGNs
([460])
while it is 20 times larger for bright
QSOs ([244]).

Despite intensive studies, the BLR is still poorly understood.

It is now widely believed that accretion of gas into a central supermassive BH
lies at the heart of the phenomenon; the accretion flow takes the form of a
geometrically thin disk which is the source of the X-ray, UV and optical
continuum emission which ionizes circumnuclear gas in both the broad-line and
narrow-line regions; the BLR is made of an assembly of small clouds,
photoionized by the continuum emission of the disk; this results from the fact
that the ``effective'' volume giving rise to the emission is much smaller than
L3, where L is the typical size of the emitting region
deduced from the
variability time-scale and the ionization parameter. However, these
requirements
can also be fulfilled if the lines are emitted by a continuous medium whose
thickness is much smaller than its lateral dimension, for instance a thin disk
or the atmosphere of a disk. Assuming that the velocity is Keplerian, FWHM of
~ 5 000-10 000 km s-1 implies that this material is located at
distances of about 103-104 RS from the
center (where
RS is the Schwarzschild radius 2GM/c2); if the
disk is heated by
the down scattered part of the non-thermal continuum observed in AGNs, the
physical parameters of the optically thin region satisfy the requirements of
photoionization models for the line emission
([88]).
In these
conditions, the low ionization lines (Balmer lines, Fe II lines, Mg II) can be
emitted mostly by the accretion disk, but not the high-ionization lines
(Ly, C IV, C III]) which are
likely emitted by a dilute outflowing
medium; this model yields BH masses in the range 107-109
M for a sample of six
Seyfert 1s
([358]).
In NGC5548, the
emission lines are variable, but are best explained by the superposition of an
emission line cloud with variable lines and another which shows no variability;
the non variable cloud may not be radiatively heated; it is in collisional
equilibrium with a temperature ~ 104 K; its emission spectrum is
dominated by Balmer lines and Fe II emission
(Dumont et al. 1998).

The traditional QSO BLR consists of two components: one with ~ 2000
km s-1 FWHM, the intermediate line region (ILR), and another
very broad
component of width > 7000 km s-1 FWHM, the very broad line
region
(VBLR); the spectra of the ILR and VBLR are very different; the high-ionization
lines are relatively stronger in the VBLR
([66]).
The ILR and VBLR are probably identical with the low- and
high-ionization line regions identified by Rokaki et al. (1992).

A highly significant correlation has been found for radio loud QSOs between the
line widths (FWHM) of broad
H lines and the
relative strength of the
compact radio nucleus; this is expected in beaming models if the predominant
motion in the line emitting gas is confined to a disk lying
perpendicular to the
radio axis which is the case if the ILR is in the thin disk
([476];
[55]).

There is an anticorrelation of the EW of broad lines (C IV
1550,
C III]1909 and Mg II
2798) with the continuum
luminosity;
these correlations are substantially stronger for the radio selected
sample than for the radio quiet sample
([399];
[343]).
This is
the so-called ``Baldwin effect''. There is no evidence for a Baldwin effect in
the broad H or
H component
([293]).

A small number of AGNs, mainly but not exclusively radio loud, have been found
to have double-peaked line profiles which could originate in Keplerian thin
disks; they have a mean FWHM of
~ 12 500 km s-1; their spectra are
characterized by large [O I]/[O III] ratios, i.e. they are
Liner-like
([109];
[179];
[405];
[355]).
In one of these objects, Arp 102B, UV spectra show broad
Mg II present with nearly the same profile as the Balmer lines, but there is
little, if any, C III], C IV or Ly
emission corresponding to the
displaced Balmer-line peaks, demonstrating the need to invoke different
locations and different physical conditions for double-peaked and
single-peaked
line components in the same object. The double-peaked component could be
the low temperature, collisionally excited region postulated by
[107].

H2O megamasers have been looked for in a number of nearby
galaxies; they
have been detected in 11 Seyfert 2s, never in Seyfert 1s; this lack of
detection
in Seyfert 1s indicates either that they do not have molecular gas in their
nuclei with physical conditions appropriate to produce 1.3 cm
H2O masers,
or that masers are beamed away from Earth, in the plane of the obscuring
molecular torus; the first possibility would violate the unified scheme
([58]).
Kartje et al. (1999) suggested that the maser emission regions
are clumpsy; when two maser clouds having the same velocities are overlapping
along the line of sight, their brightness temperature is greatly
enhanced through ``self-amplification''
([101]);
this could account for the fact that
only Seyfert 2s seen nearly edge-on are detected as then the probability of
seeing two aligned clouds is maximized
([215]).

H2O masers emitted from a rotating molecular disk with a
radius of the
order of one parsec have been observed in two Seyfert 2s:
NGC1068
([136])
and NGC4258
([168];
[295]);
both galaxies contain a massive (2-4 107
M) nuclear BH. The
discovery of these molecular
disks provides one of the most compelling evidence today for the
existence of a massive BH in the nucleus of an AGN.